The plant hormone auxin is a key regulator of plant development, with roles in meristem function, leaf initiation and vascular patterning that are conserved within the vascular plants. In Arabidopsis, many papers have identified pivotal contributions of PIN-FORMED gene family members to auxin action by regulating auxin transport either between or within cells. Whilst 'long' PINs that localize to the plasma membrane regulate intercellular transport, 'short' PINs that localize to the ER regulate intracellular auxin levels.

Contrary to previous suggestions that only ER-targeted PINs function in early diverging land plant lineages, we have shown that PINs in the moss Physcomitrella have polar localizations at the plasma membrane.

The Harrison lab has demonstrated roles for polar auxin transport in leaf development and meristem function in gametophytic leafy shoots and has also found that disrupting PIN function can lead to sporophytic branching.

This result is exciting in the context of the innovation of the earliest sporophytic branching forms in land plants, as it reproduces a form only seen before in the fossil record and rare natural moss variants, thereby suggesting a role for PIN-mediated auxin transport in the evolution of branching.